The Problem of Forecasting

"the causes of the change from a ten-year surplus of $5.6 trillion to a ten-year deficit of $2.9 trillion--a swing of $8.5 trillion. The biggest factor was increased spending, of which increased defense spending was the largest piece. The second biggest factor was changed economic and technical assumptions (that is, the forecasters were wrong)." Greg Mankiw

Oooops. Only off by $8,500,000,000,000.